The invention relates to a circuit arrangement for the drive of at least one thyristor connected to an alternating voltage. A phototransistor and an auxiliary transistor which enables an ignition of the thyristor only in a proximity of a zero point of the alternating voltage are provided.
Such a circuit arrangement has been described, for example, in "Siemens Components", 20 (1982), No. 3, FIG. 2 and pages 83-84, incorporated herein by reference. Given this circuit arrangement, the base terminal of the auxiliary transistor is connected to the alternating voltage via a resistor. The collector-emitter path of the auxiliary transistor lies in parallel with the gate-cathode path of the thyristor to be ignited. When the alternating voltage then rises from zero to a specific value, then the auxiliary transistor becomes conductive and short-circuits the gate-cathode path of the thyristor. The thyristor can then no longer be triggered. Ignition is only possible as long as the alternating voltage has not exceeded a specific value and the phototransistor lying between the base terminal and emitter terminal of the auxiliary transistor is activated.